This invention concerns a process for separating and recycling reaction byproducts in the manufacture of dianhydro sugar alcohols.
Anhydro sugar alcohols, in particular derivatives of mannitol, iditol, and sorbitol, are known for their therapeutic uses and uses in food. At least one of these, isosorbide, 1,4:3,6-dianhydrosorbitol, is useful as a monomer used in the manufacture of polymers and copolymers, especially polyester polymers and copolymers.
Anhydro sugar alcohols are produced by dehydration of the corresponding sugar alcohols (or monoanhydro sugar alcohols) by the action of various dehydration catalysts, typically strong acid catalysts.
Batch processes for the preparation of dianhydro sugar alcohols by acid dehydration are known in the art.
In particular, a batch process for the formation of the dianhydro sugar alcohol isosorbide has been described as a two step process involving intramolecular dehydration of sorbitol to 1,4-sorbitan (1,4-monoanhydrosorbitol), and further reaction of the sorbitan to isosorbide (1,4:3,6-dianhydrosorbitol) in an acid catalyzed dehydration-cyclization. This reaction produces isosorbide and a higher molecular weight byproduct. The byproduct is presumably produced by water elimination between two or more sorbitol molecules, but its exact nature is not clearly defined. See G. Flxc3xa8che and M. Huchette, Starch/Starke (1986), 38(c), 26-30 and Roland Beck, Pharm. Mfg Inc. (1996), 97-100. Other monoanhydro byproducts, 2,5-anhydro-L-iditol and 2,5-anhydro-D-mannitol, are also known to be produced under some reaction conditions (Acta. Chem. Scand. B 35, 441-449 (1981)).
WO 00/14081 describes a continuous process for producing anhydro sugar alcohols, especially isosorbide, comprising the steps of introducing at least one sugar alcohol or monoanhydro sugar alcohol into a reaction vessel; dehydrating the sugar alcohol or monoanhydro sugar alcohol in the presence of an acid catalyst and an organic solvent to form a reaction product which is at least partly soluble in the organic solvent; removing water from the reaction vessel; removing organic solvent comprising the dissolved reaction product from the reaction vessel; separating the reaction product from the removed organic solvent; and recycling the organic solvent into the reaction vessel.
U.S. Pat No. 6,407,266 describes a continuous process in which a process stream containing at least one sugar alcohol or monoanhydro sugar alcohol and, optionally, water is introduced to the first stage of a multistage reactor and then intimately contacted with a countercurrent flow of an inert gas at elevated temperature to remove the bulk of any water present to yield a dewatered process stream. This dewatered process stream is then intimately contacted with a counter current flow of an inert gas at elevated temperatures, in the presence of a dehydration catalyst, to remove water of reaction as formed. Finally, the reaction product is removed from the bottom of the reactor.
The reaction product of the above processes contains the desired dianhydro sugar alcohols and undesired byproducts as described above. The dianhydro sugar alcohols are isolated from the reaction mass by one or more purification steps such as evaporation, distillation, extraction and ion exchange or combinations thereof.
Commonly owned CL-1894, filed simultaneously herewith, teaches a continuous process for the manufacture and separation of dianhydro sugar alcohols, especially isosorbide, wherein, starting with an aqueous solution of sugar alcohol at a concentration of about 45%-50%, acid-catalyzed dehydration to form the dianhydro sugar alcohol is carried out in a single, multistage reaction-separation vessel, and separation of the dianhydro sugar alcohol from the reaction mass occurs simultaneously with the help of water vapor evolved. In the above processes, separation of dianhydro sugar alcohols from the reaction mass is not complete, and substantial quantities of product are lost with the higher boiling byproducts. Increasing the vaporization temperature may increase product recovery, but this adversely affects product quality, as more of the byproducts and color-forming impurities are vaporized along with the desired product. In the case of chromatographic separation, substantially larger equipment is required to achieve higher recovery, which may not be economical.
For isosorbide to be used as a monomer in high volume applications, such as polyester containers, there is a need for a continuous, high yield, high recovery, economical process for producing isosorbide.
The object of the present invention is to provide a process wherein the reaction byproducts and anhydro sugar alcohols not recovered initially from the reaction mass are recycled back to the reaction step and overall yield of the anhydro sugar alcohols, such as isosorbide, is increased.
This invention provides a process for separating monomeric and dimeric sugar alcohols from a mixture comprising monomeric, dimeric and polymeric sugar alcohols, comprising:
a) diluting with water a mixture comprising monomeric, dimeric and polymeric sugar alcohols;
b) allowing most of the polymeric sugar alcohols to precipitate from the diluted mixture obtained in (a); and
c) separating the precipitated polymeric sugar alcohols from the diluted mixture to obtain a solution of soluble monomeric and dimeric sugar alcohols.
The process can be conducted in a batch, semi-batch, or continuous mode. It can also be conducted with an additional step of purging a portion of the soluble monomeric and dimeric sugar alcohols.
The process of this invention can be used to separate the generally more desirable monomeric and dimeric sugar alcohols from the higher molecular weight, polymeric sugar alcohols in a byproduct stream derived from the production of a dianhydro sugar alcohol via an acid-catalyzed dehydration of the corresponding sugar alcohol. The by-product stream is that portion of the reaction mass that remains after a portion of the desired dianhydro sugar alcohol has been removed from the reaction mass, e.g., by distillation or crystallization.